1. Field of the Invention
This invention relates generally to ventilators for delivering breathing gas to the lungs of a patient, and more particularly concerns a double acting reciprocating piston ventilator having a piston that is supplied with breathing gas on each stroke of the piston, automatically charging itself whenever breath support is delivered, and requiring no retraction time between breaths. A single acting piston is also disclosed that automatically draws breathing gas from a source only during delivery of breathing gas.
2. Description of Related Art
Medical ventilators are generally designed to ventilate a patient's lungs with breathing gas to assist a patient in breathing when the patient is somehow unable to adequately breath without assistance. Pressure assistance often can be instituted when the patient has already begun an inspiratory effort, typically by bellows or fixed volume piston type ventilators.
Conventional piston lung ventilators commonly use a single action piston, in which the gas to be delivered is drawn into a cylinder by the retraction of the piston and is subsequently delivered to the patient by advancing the piston. The supply flow rate during the retraction time of a .single action piston can be much higher than the patient's peak flow demand. It would be desirable to provide a piston ventilator requiring a supply flow rate only as great as the rate required by the patient.
Such piston lung ventilators, while typically having piston seals, may allow small volumes of breathing gas with a high oxygen concentration to leak past the seals, to escape into the interior of the ventilator. A piston ventilator using an unsealed piston is also known that results in a higher volume of leakage. For example, in the known unsealed piston ventilator, the piston cylinder has a volume of about 2.7 liters, allowing for as much as 0.7 liters of compensation for leakage past the piston and volume lost due to the compressibility of the breathing gas in the cylinder and airway, while still maintaining the ability to deliver a breath of up to 2.0 liters inspired volume in one stroke of the piston. It is unsafe to allow high concentrations of oxygen to accumulate in the interior of an electrical product, due to the risk of fire, and it is therefore necessary with such single action ventilator systems to scavenge any gas mixture that has leaked past the piston outside the ventilator enclosure. The breathing gas mixture that remains in the piston to be delivered to the patient can also be diluted by the leakage, or "blow by" of room air that leaks past the piston during the time the piston is retracting, drawing breathing gas into the piston cylinder. It would be desirable to provide a piston ventilator in which any leakage of air past the piston to dilute the breathing gas mixture is avoided, and in which any leakage of the breathing gas mixture past the piston is retained within the piston cylinder, so that it does not need to be scavenged, and can be delivered to the patient.
In addition, since such single acting pistons have a fixed volume, in order for the ventilator to deliver a breath larger than the volume of the piston cylinder, the single acting piston must first advance, reverse direction, and then retract before advancing again, causing a significant interruption of flow in the time that is necessary for the single acting piston to cycle back to a "home" retracted position to deliver breath support to a patient. It would thus be desirable to provide a piston ventilator having a piston that automatically charges itself whenever breath support is delivered, and that can deliver a breath larger than the piston cylinder volume when required, without an appreciable interruption of flow, simply by reversing the direction of travel of the piston, by the use of multiple strokes of the piston, requiring no retraction time and a reversal time of only milliseconds between strokes of the piston to deliver breath support. A first embodiment of the present invention meets these needs, while a second embodiment of the invention provides gas scavenging and reduction of peak flow from the breathing gas supply.